Create connection condition between VBUP,VBFA,VBRP

Hi everyone,

i have 3 Tables with the Sequence: VBUP,VBFA,VBRP.

now i need to join table VBUP and VBFA and then VBFA and VBRP.

But i don't know which field of each table can i use to join them.

Please help

Thanks